Implantable medical devices (IMDs) are implantable or partially implantable. Some examples of IMDs include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), subcutaneous implantable cardioverter defibrillators (S-ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy, or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. The devices can be implanted subcutaneously and can include electrodes that are able to sense cardiac signals without being in direct contact with the patient's heart. Other examples of IMDs include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability (e.g., vagus nerve stimulator, baroreflex stimulator, carotid sinus stimulator, deep brain stimulator, sacral nerve stimulator, etc.).
Implantable medical devices can be programmed over wireless communication links by means of an external programming device including a transceiver. The implantable medical device can thus, when implanted, be programmed by the physician or caregiver to provide the desired function, for example adjusting the pacing mode of the pacemaker for maintaining a desired heart rate. The importance of having a reliable communication link between the implantable medical device and the external programming device is readily understood. However, the size of the medical implantable device is rather restricted and limits the size of required communication means, such as antennas.
The ability of the antenna to propagate electromagnetic waves can be dependent on the antenna shape and size as well as on the orientation of the antenna. The gain of the closed loop antenna, which is an antenna conventionally used in medical implantable devices, can be dependent on the area enclosed by the antenna wire and the loop antenna radiation pattern thus depends largely on the size and the orientation of the loop formed by the closed loop antenna. As the size of implantable medical devices or portion of the implantable medical device housing the antenna decreases, maintaining or keeping the area enclosed by the antenna as large as possible can become difficult.